Electrical transmission system



L. E. DEERING ELECTRICAL TEANSMISS ION SYSTEM Filed May 15, 1925 w, H .m A W n w m f m e m w a M Q R QM, \N WMQ W+b Patented Aug. 20, 1929.

UNITED STATES PATENT OFFICE.

LAWRENCE E. DEERING, OF HOLLIS CENTER, MAINE, ASSIGNOR, BY MESNE ASSIGN- MENTS, T0 WESTERN ELECTRIC COMPANYQINCORPORATED, A CORPORATION OF NEW YORK.

ELECTRICAL TRANSMISSION SYSTEM.

Application filed Harm, 1925.

This invention relates to electrical transmission systems and more particularly to :a method of and means for obtaining good quality and efiicient transmission of a wave band in repeater systems.

Repeaters have been employed in systems of various types, for example, in ordinary long line telephone systems, to repeat voice frequency waves, and in carrier systems, using carrier waves of different frequencies.

W hen it is desired to use a repeater to obtain high amplification, it has been the practice to employ a plurality of cascade connected amplifying units which may be 4 coupled in various manners.

It is well known that the selectivity and efficiency of a multi-stage repeater or amplifier may be materially improved by coupling the successive stages by means of selective circuits tuned sharply to the mid-frequency of the wave band to be repeated. However, when such systems are used to repeat a frequency band of considerable width, it has been found that the sharply tuned circuits introduce considerable distortion. This is especially true in carrier systems in which the repeated wave comprises the unmodulated carrier component and both side-band components. In this case the carrier frequency proper and those' portions of the side-bands which are closely adjacent to the carrier frequency will be faithfully repeated, whereas the frequencies of the more remote components of the side-bands will be distorted.

Another difiiculty met within such circuits, especially when sharp tuningis resorted to, is the tendency for energy to be fed-back from the output to the input circuit of the repeater. As is well known this regenerative action tends to cause the production of oscillations of undesired frequencies or singing in the circuit and hence serves to limit the degree of amplification which can be satisfactorily used. Again, while the amplification factor of each amplifier unit may be kept low enough to prevent singing at any frequency lying within the frequency band to be repeated, the system, in-

- cluding the complete series of units, will Serial No. 29,884.

which may be used to sharply select and highly amplify a wave comprising a relatively wide band of frequencies, without causingdistortion of any of the current components lying within the frequency band to be repeated.

In accordance withthis invention the wave to berepeated is supplied to a selective circuit tuned sharply to its mid-frequency, and the'repeater system is providedwith means to compensate-the distortion introduced by the sharply tuned circuit.

The compensating device'to be hereinafter described, will preferably comprise an electrical network resonant at the mid-frequency of the band and having impedance characteristics for the frequency band to be repeated, which are the exact reciprocal of the distorting means to be compensated for.

It hasbeen found that by properly placing a network of this t pe in the repeater system, the tendency of the system to sing may be greatly reduced, thus permitting an increase in the overall amplification of the system.

One object of this invention is to secure high selectivity andv good quality transmission in systems of the type described above.

Another object is to secure sharp selectivity while permitting extremely high amplification to be used without causing the repeater system to sing.

A'feature of the invention is to provide a simple means adapted to simultaneously compensate the distortion introduced by the sharply tuned circuit and to reduce the tendency of the repeater system to sing.

In order to disclose a practical application of the invention it will be describedas-applied to the intermediate frequency amplifier of "a radio broadcast receiving system employing successive detection. The invention is particularly applicable to broadcasting systems, since the requirement as to quality and efliciency are especially exacting in systems of that type. i

For a complete understanding of the invention and the operation of its various parts reference should bemade to the following specification read in conjunction with the attached drawing in which: Fig. 1 shows a radio'receiving system to which this inventionhas been applied; Fig. 2 is a diagram which will be used to explain the operation of the sharply tuned circuits; and Fig. 3 shows one form of compensator or equalizer which may be used.

Fig. 1 illustrates a radio receiving system 5 employing double detection, but such details as have no bearing upon the invention have been omitted.

The system as shown consists of a high frequency receiving or antenna circuit 10, a first detector 11 having an oscillator 51 associated with it, three tandem connected intermediate frequency amplifiers 12, 13 and 14, a'

second detector 15, an audio-frequency power amplifier 16 of the push-pull type, and a voice frequency receiving device 17, which may be a telephone receiver or loud speaker.

The detecting and amplifying devices are all shown, by way of example, as three ele: ment space discharge devices provided with 20 the usual sources for supplying heating cur- .-rent to their cathodes and space current potential to their anodes. Transformers 21 to 26 serve to inductively intercouple the several stages in the well knownmanner.

In order to facilitate the description of "the operation of the system, let it be assumed that a carrier wave 0, of 800,000 cycles, modulated by speech frequency waves of frequency s, of 200 cycles to 3,000 cycles, is re- 30 ceived by the antenna circuit 10. The frequency of the modulation products of the carrier wave, or side band components, will then be represented by 800,000 i s. The complete wave will embrace a frequency band extending, for example, from 803,000 cycles to 797,000 cycles. The antenna circuit 10 is sharply tuned to 800,000 cycles in any well known manner, a variable condenser 30 being shown as a desirable tuning control 40 means.

Energy of the incoming wave is trans ferred to the secondary winding of transformer 21, which together with condenser 31, forms a selective circuit. By adjusting the condenser 31 this circuit is sharply tuned to 800,000 cycles. The modulated carrier may be assumed to be impressed upon the grid circuit of tube 41 without any extraneous frequencies.

" is used to supply oscillations of 750,000 cycles through transformerv 52 to the grid circuit of tube 41. Due to the interaction endary winding of transformer 22 and the An oscillator 51 of any appropriate type condenser 32, which is tuned sharply to 50,- 000 cycles.

The modulated intermediate frequency carrier wave of 50,000 i s, i. e. 53,000 cycles to 47,000 cycles, is amplified by the amplifier consisting of three stages 12, 13 and 14, which are intercoupled by the transformers 23 and 24. The secondaries of these transformers may, if desired, be bridged by tuning condensers similar to the condenser 32 and tuned to 50,000 cycles, in order to exclude oscillations of harmonically related and other undesired frequencies produced in the amplifier circuits.

The output circuit of amplifier 14 includes a compensating device 53 the purpose of which will be described below.

The amplified modulated wave is transmitted through transformer 25 to the selective circuit, formed by the secondary winding of this transformer and condenser 35, which is tuned to 50,000 cycles, and impressed upon the input or grid circuit of detector tube 45. The usual leak path comprising a condenser shunted by a resistance is associated with the input electrodes of the detector 45. Due to the well known detecting action of tube 45, the speech wave will be produced in the output circuit of the tube and transferred by the transformer 26 to the audio frequency amplifier 16, which may be of the well known push-pull type, disclosed in U. S. Patent No. 1,128,292, February16, 1915, to Colpitts. The amplified speech wave will be supplied through transformer 27 to the receiving device 17.

In Fig. 2 is shown the resonance curve of a selective circuit, such as 21-31, 22-32, or 25-35 in Fig. 1. 7 Assuming that the average current in theprimary winding of the transformers 21, 22 and 25 is constant and that the selective circuit including the secondary winding is tuned to the frequency C, as the applied frequency departs from the frequency C, the value of the potential of different frequency waves across the tuned circuit 2131, 2232 or 2535 will vary in accordance with the curve. Therefore, if a modulated wave of constant amplitude and comprising an unmodulated component of frequency, 0 and-side bands C s and C s are supplied to the primarywinding, the'potentials in the tuned. circuit, induced by the freqeuncy components C s and C s, will be represented by points on the curve at the right or left of the line 00, for example, by OM and ON.

Thus, the amplitudes of the ditferentfrequency components present in the tuned circuit will differ, and the modulated wave, as a whole, will be distorted.

Considering the effect of such distortion in the system shown in Fig.1, it will be apparent that in the case of tuned circuit 2131,

where the frequencyof the wave is high and the frequency band to be selected is relatively narrow, the points M and N will lie closely adjacent to and substantially upon the flat top portion of the curve and no appreciable distortion will take place. However, where the frequency of the wave is reduced to 50,000 cycles, as in the case of the tuned circuits 2232 and 25-35, the distortion becomes pronounced in each circuit and for the two circuits, its effect is additive.

The operation of the compensator or equalizer 53 will now be explained. One embodiment of such compensating device is shown in Fig. 3. It consists of a variable condenser 71, a variable inductance 72 and a variable resistance 73 forming a series circuit. This circuit is connected between conductors 74 and 75 of the output circuit of the amplifier 14 in Fig. 1. This series circuit forms a more or less effective shunt for the carrier wave traversing the conductors 74 and 75. Assuming a constant average potential for the repeated wave and proper tuning of the series circuit 73727 1, the current fiowing therethrough will vary with the frequency in accordance with the curve shown in Fig. 2, that is, more current will be shunted from the line 74-75 at the base frequency C than at the limiting frequencies C s and C s of the side bands.

It will therefore be apparent that, by properly designing the impedance characteristics of the compensating or equalizing circuit 58 and tuning it to 50,000 cycles, the value of the currents of carrier and adjacent frequencies traversing the circuit 74.75 may be adjusted to compensate the excess currents of these frequencies due to the distorting effects of the system. And likewise, since the resistance 7 3 may be adjusted to more or less flatten the top of the resonance curve of the equalizer, it will be possible to entirely compensate excess currents of any particular frequency lying within the repeated band, whether produced by the distorting effect of the tuned circuit 22-32 or other causes.

Furthermore, since the wave will be subjected to further distortion by the tuned circuits 2535, the equalizer 53 may be adjusted to withdraw enough current at the carrier and adjacent frequencies to compensate the subsequent distorting effect of the selective circuit 2535.

The compensating device may be of any type which will have an equalizing effect upon the different components of the carrier wave, and it may be placed at a number of the points, for example, these marked XX, Y-Y or ZZ in Fig. 1. It is also feasible to employ more than one equalizer in the system. For example, one may be placed at each of the points XX, Y-Y or ZZ to compensate the distortion produced by the respective units 12, 13 and 14.

In systems in which a plurality of stages are used to amplify successively different intermediate frequencies, i. e. where the frequency is reduced in more than one step, it is possible to employ either a single equalizer in some one stage to compensate for the total distortion of all stages or else to locate an equalizer in each of the stages to compensate the distortion introduced by the circuit with which it is associated.

It is to be understood that this invention is not limited in its application to the particular system described above, nor to compensating means embodying the specific details herein set forth. The invention is furthermore adaptable not only to radio systems, but its principles are applicable to carrier line and other systems in which a wave, comprising a band of frequencies of appreciable width, is to be repeated by a system including circuits so tuned as to distort certain of the current components lying within the band to be repeated.

lVhat is claimed is:

1. The method of receiving a modulated carrier wave comprising rendering the carrier wave readily amplifiable by reducing its frequency, selecting the reduced frequency carrier waveby tuning to one component thereof, amplifying said reduced wave to a high degree, selecting the reduced frequency wave by tuning to one component thereof, compensating the distortion of said successive selections, detecting the compensated wave, and operating an indicator by the current resulting from said detection.

2. In a carrier current telephone system including a repeater employing high resonant tuning and high amplification, the method of faithfully reproducing the speech wave which comprises simultaneously compensating the distortion of the carrier Wave caused by said tuning and counteracting the tendency, due to said amplification, to cause singing in the repeater, and detecting the compensated Wave.

3. A repeater for faithfully reproducing low frequency signal currents represented b modulations of a carrier wave including di ferent frequency components, one component being unmodulated carrier frequency comprising means for selecting the wave, a circuit having dissimilar impedances for the different current components of the wave, means for impressing the wave upon a second circuit having for said different current components dissimilar impedances which are the reciprocal of those of the selective circuit, and means for detecting the Wave supplied by the second circuit to produce said signal currents.

4. A receiving system for a carrier Wave modulated in accordance with signals comprising means for reducing the frequency of the modulated Wave, means for selecting the reduced frequency wave comprising a circuit tuned to the unmodulated component thereof, means for amplifying said reduced frequency wave including means for simultaneously compensating the distortion intro duced by said tuned circuit and preventing singing in said amplifier, and means for detecting the compensated wave to yield the signals. 7

5. A repeating system comprising a source of signal modulated waves including side bands and an unmodulated carrier component, a selective circuit tuned to the frequency of said carrier component and having different impedance to said carrier and side bands a space discharge tube connected to said circuit for repeating said modulated Waves, and a path, including an adjustable resistance and a plurality of adjustable reactances in series, adapted to offer to said repeated waves an impedance which is the reciprocal of that offered by said selective circuit.

6. A system for amplifying a wide band of waves including a signal modulated carrier wave, comprising a circuit for selecting said band to the substantial exclusion of waves of other frequencies, said circuit producing a greater loss at some frequencies of said band than at others, an electric discharge device connected to said circuit for amplifying said band, and an impedance network associated with said discharge device and having a loss characteristic which is the reciprocal of that of said selective circuit for the frequencies in said band.

7. A receiving system for a wide band of waves representing a modulated carrier wave comprising means for reducing the frequency of said band means for selecting the reduced frequency band to the substantial exclusion of waves of other frequencies, said means producing a greater loss at some frequencies of said reduced frequency band than at 0th ers an electric discharge device for amplifying said reduced frequency band, and an impedance network associated with said dis charge device and having a loss character istic which is the reciprocal of that of the selective means for the frequencies in said reduced frequency band.

In witness whereof, I hereunto subscribe my name this 21st day of April A. 11-1925.

LAWRENCE E. DEERING. 

